High intensity electron beam generator

ABSTRACT

A high current electron beam generator including an anode and a cathode located within a vacuum enclosure. Both the anode and the cathode have a generally conical shape with the cathode being located inside the anode. Alternately, a coaxial alignment may be employed. Upon the application of a high voltage signal across the anode and cathode and due to the particular configurations used, electrons emitted from the cathode are forced to flow for the most part along rather than across equipotential lines of the electric field. This parapotential flow produces a high intensity electron beam which can be subsequently focused for use in various applications.

United States Patent represented by the Secretary of the Navy [54] HIGH KNTENSITY ELECTRON BEAM HIGH VOLTAGE SOURCE VACUUM ENCLOSURE 11/1958 Hansen.... 3/1965 Dyke et al OTHER REFERENCES Grayhill & S. V. Nablo, Techniques for the Study of Self- Focusing Electron Streams, Proceedings of the 8th Annual Electron & Laser Beam Symposium, April, 1966, p. 465-485 (328/228) Primary Examiner-David Schonberg Assistant Examiner-Paul A. Sacher Attorneys-R. l. Tompkins, Arthur L. Branning and R. J.

Erickson ABSTRACT: A high current electron beam generator including an anode and a cathode located within a vacuum enclosure. Both the anode and the cathode have a generally conical shape with the cathode being located inside the anode. Altemately, a coaxial alignment may be employed. Upon the application of a high voltage signal across the anode and cathode and due to the particular configurations used, electrons emitted from the cathode are forced to flow for the most part along rather than across equipotential lines of the electric field. This parapotential flow produces a high intensity electron beam which can be subsequently focused for use in various applications.

ELECTRON BEAM mimosa new 1 3,603,838

[1o an HIGH VOLTAGE sounca CATHODE 22 ANODE vAcuuM ENCLOSURE 141 we" 0 VOLTAGE SOURCE ELEcmEN BEAM 7 i OATHODE k 22 move J vAcuum ENCLOSURE INVENTOR @A W 6. daPACKl-I ATTORNEY HIGH INTENSITY ELECTRON BEAM GENERATOR STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION The present invention relates to electron beam generators and more particularly to an electron beam-forming vacuum diode employing generally cylindrical anode and cathode elements, the cathode being axially aligned within the anode.

It has long been recognized that in describing electron flow in conventional low-power vacuum diode devices, a valid approximation can be made by neglecting the self-magnetic field generated by the electron beam as it traverses the space between the cathode and anode elements. As technology has progressed, however, the development of high-power vacuum diodes has become increasingly important. For such devices, the self-magnetic field cannot be neglected and, in fact, forms the limiting case in considering the construction of high-power electron beam generating diodes. While scientific principles, theories and formulas have been extensively discussed in recent years, a true understanding of electron flow in highpower vacuum diodes has been heretofore unavailable to aid the design engineer in the construction of a useful device employing such principles. As a consequence, many devices presently in use were designed primarily through the application of low-power principles with certain modifications, the exact nature of the electron flow being thus based to a large extent on conjecture rather than applicable theory. Although such devices have served the purpose, they have not proved entirely satisfactory under all conditions of service for the obvious reasons that considerable difficulty has been experienced in directing, focusing and controlling the resultant electron beam output and in mechanically constructing an efficient and effective device.

OBJECTS OF THE INVENTION It is, therefore, an object of the present invention to provide a vacuum diode capable of producing high intensity electron beams.

Another object of the present invention is the provision of a vacuum diode for producing high intensity parapotential flow.

The present invention has a further object in the provision of a high intensity electron beam generator having self-focusing characteristics.

A further object of the invention is to provide a diode device for producing electron flow along rather than across equipotential lines of the electric field.

SUMMARY OF THE INVENTION The general purpose of this invention is to provide an electron beam generator which embraces all the advantages of similarly employed prior art devices and possesses none of the aforedescribed disadvantages. To attain this, the present invention comprises a high current electron beam generator comprising an enclosure defining a vacuum, a cathode mounted within the enclosure, and an anode mounted adjacent the cathode within the enclosure and forming an electron beam gun therewith. The electron beam gun is adapted to produce a high current parapotential flow therein upon the application of a voltage having a value at least equal to a predetermined minimum value. The generator further includes a circuit coupled to the electron beam gun for generating a voltage having a value of at least the predetermined minimum value.

Other objects, advantages and novel features of the invention will become more fully apparent .from the following detailed description of the preferred embodiment of the invention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I shows a cross-sectional view of a first embodiment of the present invention;

FIG. 2 shows a cross-sectional view of a second embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, which illustrates the preferred embodiment of the present invention, there is shown a vacuum enclosure 10 which may be any conventional structure sufficient to form a vacuum chamber and having at least one portion forming a window 12 to permit a generated electron beam 14 to pass therethrough. Mounted within the vacuum enclosure I0 is an anode l6 and a cathode 18. Both the anode I6 and the cathode I8 are shaped to form two axially aligned concentric conical sections. The anode and cathode are each connected through suitable electrical conduction paths to a high voltage source 20 having a sufficiently high voltage output to generate the desired high intensity electron beam 14. The source 20 may be any conventional high voltage power source, such as a pulsed generator, the particular design characteristics being dependent upon the particular application contemplated.

In the device as shown in FIG. 1, if a potential sufficient to produce space-charge-limited emission is applied by source 20 between the cathode 18 and the anode 16, the electrons will begin to flow from the cathode to the anode directly across the space therebetween due to the electric potential difference produced by the source. At this point, the low voltage from source 20 produces a weak flow of electrons from the cathode to the anode. The flow is controlled almost entirely by the electric field across the space therebetween, and any effects which the magnetic field have on the electron flow caused by self-magnetic field generation can be neglected. As the potential output of source 20 is increased, however, the number of electrons flowing across the gap increases and the selfgenerated magnetic field begins to have a marked effect on the resultant electron flow path. As the: source 20 is increased even further, a limiting case is approached in which substantially the entire flow of the electrons within the spacing between the cathode 18 and the anode 16 is controlled by selfmagnetic field generation. The resultant electron flow is as shown in FIG. I by dashed lines 22. As can be seen in the figure, the electron flow is along rather than across equipotential lines. The resultant electron beam flow can therefore be termed parapotential flow.

As shown by the travel path of the electrons, illustrated as 22, the establishment of parapotential electron flow when properly generated, is especially adapted for the production of concentrated electron beams. In the structure of FIG. 1, which defines the preferred embodiment of the present invention, the electrons which are emitted from the cathode 118 are directed by self-generated magnetic fields towards the apex of the conically shaped elements whereupon they are automatically focused for emission through the window 12 in the vacuum enclosure 10 as a concentrated! high intensity electron beam 14. Prior to the discovery of parapotential flow and its theoretical basis, the construction of such devices as illustrated in FIG. I was not considered by engineers working in this field since it appears from a casual glance at the illustrated structure that, in accordance with conventional theories relating to electron flow indiode devices directly from the cathode to the anode, such a structure would would be entirely impractical. Quite to the contrary, however, the device illustrated herein, being capable of establishing and maintaining parapotential electron flow, produces high current electron beams in a simple, economical and efficient manner.

Referring now to FIG. 2, which illustrates an alternative embodiment of the present invention, a vacuum enclosure I0 is shown again containing a window or aperture segment 112 for permitting the emission of a generated electron beam 14. The vacuum enclosure contains an anode element 16 and a cathode element 18 mounted in an axially aligned fashion with the cathode 18 substantially surrounded by the anode 16 as illustrated. In this embodiment, both the cathode and the anode are substantially cylindrical in shape throughout their entire length. As in FIG. 1, the cathode and anode are both coupled through suitable electrical coupling means to a high voltage source 20. The source 20 produces a high voltage signal which generates the electron flow between the cathode and the anode and in the limiting case will produce parapotential flow in a manner similar to that described with respect to FIG. 1 above. It is noted however, that cathode 18 has a conically shaped segment 24 at the output end thereof. This shape is not necessary and is illustrated herein solely for the sake of completeness and to illustrate the self-focusing characteristics which can be obtained by varying the shape of the output or terminal portions of the anode and cathode to suit various particular installations.

In describing the operation of the devices shown in FIGS. 1 and 2, reference was made to the fact that the output signal from source 20 when increased to a predetermined minimum value would cause the generation of a parapotential electron flow within the cathode-anode gap. The application of the predetermined minimum value in conjunction with the unique anode-cathode structure illustrated therein produces the high intensity output electron beams 14 for subsequent use as desired. To clarify the disclosure of the present invention it is considered pertinent to describe the theoretical basis upon which the parameter values of input voltage as well as cathode-anode spacing derived.

Considering only the simplest conditions, in which all accelerations are neglected and in which axial symmetry is maintained throughout, the motion of an individual electron can be considered to be confined to a specific plane within the spacing between the anode and the cathode. it can then be shown that the potential is governed by the equation WV: 2EI? /d R is the polar radius, V is the potential, and I is the current contained within the equipotential surface defined by V. The unknown function dF/dV is partially determined by the necessity for introducing the parapotential condition E=BH after one integration of this equation.

At least two typical cases have been examined, one in which the z-variation is negligible, and the other in which the R- variation is negligible within the R-z plane under consideration. In the former case we write:

R"5/8R(R8y/ 8R)=R"dp./dy. where y is the total electron energy in me units and p. is the current in units of 17 kA. After one integration we have:

y/ (I o where t is in general an arbitrary function of z. In the present notation the condition E=BH becomes:

8y/8R=2flp./R, that B=1y"=l 2 /p., or

FF P'o and dp.=/dy=2p y; thus the original equation is linear. The complete solution is:

#o g( which involves two arbitrary functions of z, r and 1. On R=r the electron energy and the electric field are zero, and therefore the interpretation of ;1. is that it is a current which must flow within the cathode surface to produce the required conditions of the flow. If a is larger than that value which just satisfies (1) with ya (the applied anode potential) and R=r,, (the anode radius), one requires A y cosh (2m, 1 g R/r A (y -fl/(R log r,,/R)=( 2u /R) sinh (2;.t log R/r in order to satisfy the potential and field continuity conditions at the beam surface, where conditions are denoted by the circumflex. If t is less than this critical value, the flow is impossible. If [L is equal to the critical current given by p (arc cosh y,,)/2 log (r /r volt-ampere characteristic (conductance in mhos) is This or higher conductances may be regarded as representing parapotential flows. The internal current is included in this value since it is generally supplied by the same source as the sheath current and results in electrons of the same energy. In nonrelativistic flows the internal, or core, current makes up most of the flow; the opposite holds for highly relativistic flows. In the particular case u C the minimum conductance occurs for y,,- 2.5, and the corresponding conductance is 0. l74/log (r /r mhos. This minimum is very broad, and little error is incurred over the range 250 kev. V 2 Mev. if this figure is used.

In the alternative case where the R-variation can be neglected it is readily shown that the complete solution is H which involves the two arbitrary functions of R, X and a which in turn depend on the cathode and beam surface shapes. If the cathode is at 2.,(R) and the beam surface is at 2 a then z =XR and 2z,,=(R/u arc cosh y. [n the case of a metallic cathode 11. is constant, so that a constantYimplies z-z,,gR.This is an obvious limiting case of the coaxial flow just discussed, in which the separation of the equipotentials is simply proportional to R. The ratio d,,/r continues to play the role of an impedance, which can from previous considerations be taken to be the order of 5.74 d lr ohms.

This is the highest impedance giving true parapotential flow for this geometry. This concept furnishes a convenient way of matching the diode to a given source impedance. The taper is likewise useful in concentrating the flow and confining the angular spread of the beam.

Thus, there is shown and described a structurally simple, yet efficient, vacuum electron beam generator diode for producing a high intensity electron beam by establishing and effectively employing parapotential electron flow; that is, the flow of electrons along rather than across equipotential lines of electric field due to the self-generated magnetic field induced by the emitted electrons themselves.

It should be understood, of course, that the foregoing disclosure relates to only the preferred embodiments of the invention and that numerous modifications alternations may be made thereto in light of the above teachings.

What is claimed and desired to be secured by Letters Patent of the United States is:

1. An electron beam gun means for producing a high current parapotential flow comprising:

an enclosure defining a vacuum;

a cylindrical cathode mounted within said enclosure;

a cylindrical anode axially mounted around said cathode within said enclosure;

means coupled to said electron beam gun means for generating a voltage to said electron beam gun for producing said high current parapotential flow between said anode and said cathode said enclosure further including a window for allowing the produced parapotential current flow to pass therethrough; and

said anode and said cathode comprise axially aligned conical portions terminating at said window.

2. An electron beam gun means for producing a high current parapotential flow comprising:

an enclosure defining a vacuum;

a cylindrical cathode mounted within said enclosure;

a cylindrical anode axially mounted around said cathode within said enclosure;

means coupled to said electron beam gun means for generating a voltage to said electron beam gun for producing said high current potential flow between said anode and said cathode;

said enclosure further including a window for allowing the produced parapotential current flow to pass therethrough; and

said cathode comprises a conical portion terminating at said window. 

1. An electron beam gun means for producing a high current parapotential flow comprising: an enclosure defining a vacuum; a cylindrical cathode mounted within said enclosure; a cylindrical anode axially mounted around said cathode within said enclosure; means coupled to said electron beam gun means for generating a voltage to said electron beam gun for producing said high current parapotential flow between said anode and said cathode said enclosure further including a window for allowing the produced parapotential current flow to pass therethrough; and said anode and said cathode comprise axially aligned conical portions terminating at said window.
 2. An electron beam gun means for producing a high current parapotential flow comprising: an enclosure defining a vacuum; a cylindrical cathode mounted within said enclosure; a cylindrical anode axially mounted around said cathode within said enclosure; means coupled to said electron beam gun means for generating a voltage to said electron beam gun for producing said high current potential flow between said anode and said cathode; said enclosure further including a window for allowing the produced parapotential current flow to pass therethrough; and said cathode comprises a conical portion terminating at said window. 